The same pair of video codecs won't magically transcode orders-of-magnitude faster on newer CPUs†, because video codecs—like crypto primitives, compression algorithms, and a few other categories—are designed, in the choices of the types of maths they do, to take advantage of the existence of certain accelerated instructions (e.g. SSE) that exist at the time of their creation. You can't make them take advantage of newer accelerated instructions (e.g. AVX-512) without it being a different algorithm.

In other words: new CPU architectures don't make old algorithms faster†. Instead, new CPU architectures enable newer, faster algorithms.

† Yes, this used to happen, in the 90s, when "new CPU" meant "appreciably higher clock speed." Nobody is making computers faster by increasing clock speeds any more, because higher clock speeds = higher power-draw and higher TDP. CPUs are now advanced by making them work smarter, not harder: accomplishing the same abstract tasks faster by providing hardware that enables newer algorithms to be used for those tasks, while using the same or less energy than the previous implementation would have used.